The present invention relates generally to ring laser gyros. In particular, the present invention relates to a dither control system for a ring laser gyro.
Ring laser gyros, also often called laser gyros or laser angular rater sensors, are well known. One example of a ring laser gyro is disclosed in U.S. Pat. No. 4,751,718 issued to Hanse, et al., which is incorporated herein by reference. Present day ring laser gyros include a thermally and mechanically stable laser block having a plurality of formed cavities for enclosing a gap. Mirrors are placed at the extremities of the cavities for reflecting laser beams and providing an optical closed-loop path.
Associated with such laser gyros is an undesirable phenomenon called lock-in. During lock-in the counter-propagating laser beams tend to lock together to a common frequency. The lock-in phenomenon has been typically addressed by rotationally oscillating or dithering such laser gyros. The rotational oscillation is typically provided by a dither motor.
Known dither motors, such as those shown in U.S. Pat. No. 4,751,718, often have a suspension system which includes, for example, an outer rim, a central hub member and a plurality of dither motor reeds which project radially from the hub member and are connected between the hub member and the rim. Conventionally, a set of piezoelectric elements which serve as an actuator is connected to the suspension system. When actuated through the application of an electrical signal to the piezoelectric elements, the suspension system operates as a dither motor which causes the block of the laser gyro to oscillate angularly at the natural mechanical resonant frequency of the suspension system. This dither motion is superimposed upon the inertial rotation of the laser gyro in inertial space. Such dither motors may be used in connection with a single laser gyro, or to dither multiple laser gyros. Various approaches to recover inertial rotation data free from dither effects are also known.
It is also known that if the laser gyro is dithered through a constant angular displacement at a constant frequency and amplitude, xe2x80x9cdynamicxe2x80x9d lock-in effects, in which the propagating beams lock at a common frequency that is a function of the dither frequency will occur. By modulating the dither drive with random noise, the dynamic lock-in effects can be reduced. Moreover, by modulating the dither drive with random noise, the small errors that are introduced each time the gyro reverses direction when dithered can be reduced. Laser gyros using noisy dither are disclosed in U.S. Pat. Nos. 5,249,031 and 5,225,889, both issued to Fritze et al. These gyros include a dither pickoff that measures angular displacement of the gyro block and from which a dither pickoff signal representative of the measured angular displacement is generated. An error signal is calculated from the difference between a desired angular displacement and the dither pickoff signal. The error signal is then modulated with random noise, and the resulting signal is used to drive the dither motor. Also, in some conventional laser gyros, the error signal is integrated in order to generate (for instrumentation purposes only) a signal representative of the average drive signal provided to the dither motor.
In order to randomly dither the gyro block to properly reduce dynamic lock-in effects, that part of the drive signal corresponding to the noise will be significantly larger than the part corresponding to the error signal. For example, the drive signal ultimately applied to the dither motor can have a peak-to-peak range of about 300 V. In order to randomly dither the sensor, about 100 V of the drive signal""s amplitude range is needed for adding noise. Under normal operating conditions, the error signal component of the drive signal has a peak-to-peak amplitude of about 20 V. Under such conditions, a sufficient amount of the drive signal""s amplitude range is available to accommodate the noise component. However, under lossy conditions (for example, when the gyro is operated at relatively low temperatures), the error signal component of the drive signal can become so large that an insufficient amount of the drive signal""s amplitude range is available to accommodate the noise component fully. This condition is referred to as xe2x80x9csaturationxe2x80x9d of the dither drive, and reduces the effectiveness of noisy dither in reducing errors that are introduced due to dynamic lock-in effects.
There is a continuing need, therefore, for improved dither control systems that reduce the occurrence of dither drive saturation.
The dither control system of the present invention reduces the command angle in order to help avoid saturation when the command angle becomes too high. Moreover, the average drive that is output by the system to satisfy the drive requirements are read with more accuracy and the errors that occur due to increased drive requirements at low and high temperatures can be reduced with the dither control system of the present invention.
The dither control system of the present invention can be used in a dithered ring laser gyro having a pickoff source for providing pickoff data representative of a measured dither angle and a command angle source for providing command angle data representative of a desired command angle. The dither control system comprises means for generating modulated data as a function of dither error data and noise. The dither control system further comprises means for generating average drive data as a function of the dither error data and means for generating dither drive data as a function of the modulated data. The dither control system also includes a maximum drive source for generating maximum drive data as a function of a maximum desired value of the average drive data and means for generating command angle reduction data as a function of the average drive data and the maximum drive data. In addition, the dither control system includes means for generating the dither error data as a function of the pickoff data, the command angle data, and the command angle reduction data.
In one embodiment, the dither control system of the present invention includes electronic memory for storing data, including software, and a digital controller coupled to the pickoff source and the electronic memory. In this embodiment, the command angle source, the modulated data generating means, the average drive data generating means, the dither drive data generating means, the maximum drive source, the command angle reduction data generating means, and the dither error data generating means include software that is stored in the electronic memory and is executable by the digital controller.
Also, a dither control method is provided by the present invention. The dither control method can be used with a dithered ring laser gyro having a pickoff source for providing pickoff data representative of a measured dither angle and a command angle source for providing command angle data representative of a desired command angle. The dither control method of the present invention comprises generating modulated data as a function of dither error data and noise. The dither control method also includes generating average drive data as a function of the dither error data, and generating dither drive data as a function of the modulated data. In addition, the dither control system includes generating maximum drive data as a function of a maximum desired value of the average drive data, generating command angle reduction data as a function of the average drive and the maximum drive data, and generating the dither error data as a function of the pickoff data, the command angle data, and the command angle reduction data.
FIG. 1 is a block diagram of a laser gyro in which a dither control system of the present invention can be implemented.
FIG. 2 is a block diagram of a dither control system of the present invention.
FIGS. 3-6 are high-level flow diagrams of software for implementing portions of the dither control system of the present invention in the laser gyro of FIG. 1.